Amplifying system



July 29, 1930.

B. "BRADBURY 1,771,913

AMPLIFYING SYSTEM Filed April 7', 1928 Inventor: B uwk e Bradb wry by His Attorney Patented July 29, 1930 UNITED STATES PATENT OFFICE BURKE BRADBURY, OF SCHENECTADY, NEW YORK, ASSIGNOR '10 GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK AMPLIFYING SYSTEM Application filed April 7, 1928.

The present invention relates to amplifying systems, and more particularly to systems employing electron discharge apparatus for the amplification of high frequency currents.

In amplifying systems involving the use of the three-electrode, electron discharge de vices it has been proposed to tune the output circuit for the purpose of obtaining selectivity but this method of obtaining selectivity has not been generally used because of the fact that the output impedance of the device is low enough to reduce greatly the sharpness of the tuning which can be obtained. The use of a resonant output circuit also tends to the production of oscillations because of the feed-back through the device due to the capacity between the grid and plate electrodes.

To avoid these disadvantages it has become customary to use an untuned output circuit loosely coupled by a step-up transformer to a tuned input circuit of the succeeding device. By this expedient the eftect of the output impedance of the device in decreasing the sharpness of the tuning has been very greatly decreased and at the same time the tendency to oscillate has been decreased.

It is known that with a fourelement electron discharge device having two grids, one of which serves to screen the plate and control. grid to eliminate the effect of capacity coupling between the two the plate or output impedance is many times greater than that of an ordinary three-element device. Because of this difference in the output impedance it has been proposed to provide a tuned output circuit for the device as good selectivity may be obtained by this expedient, and at the same time the tuned output circuit provides a high external impedance in the output circuit which is conducive to efiieient operation. This arrangement however has the disadvantage that where the system is employed for operation over a wide range of Serial No. 268,356.

frequencies the impedance of the tuned output circuit will vary considerably over the range of frequenci s be used and hence uniform amplification cannot be obtained over the entire frequency range.

a circuit arrangement which will overcome this disadvantage, which will furnish sufficiently high output impedance to insure a high degree of amplification and which at the same time will give a substantially uniform degree of amplification over the entire operating range of the apparatus.

I11 carrying my invention into effect I provide tuned input circuits for one or more electron discharge amplifiers and closely couple these tuned input circuits by transfomers to the untuned output circuits of the preceding four electrode screen grid electron discharge devices.

I have found that these transformers may to advantage have a one to one ratio of turns, or may even have a greater number of turns in the primary winding than in the secondary. Vith this expedient the effective impedance of the output circuit of th four-electrode device maybe made substantially as high or even higher than would be the case if the output circuit itself were tuned. lVhere a greater number of primary than secondary turns are employed the effective impedance of the output circuit may be made somewhat higher than if the ratio were held at one to one and this may in some cases appreciably increase he degree of amplification obtained as the plate impedan e of the four-element tube is ordinarily considerably higher than the im pedance at resonance of a well designed parallel tuned circuit and it is well known that for the maximum degree of annalification the output. impedance of the circuit should be substantially equal to the tube impedance.

I have also found that by suitable sepa: tion of the secondary and primary of the transformer the capacity between the winda if? The object of my invention is to provide Capacity between windings- 406111.111. f. Coefficieut of coupling 99.4%

Frequency Amplification 600 K. C. 28.5 1000 K. C. 31.5 1400 K. C. 36.0

In comparison with this another transformer was wound with .020" of paper insulation between windings and the measurements were as follows:

Capacity between windings-" 106111.111. f. Coeflicient of coupling 03.2%

Frequency Amplification 600 K. G. 27 1000 K. C. 30 1400 K. C. 29

In the latter case it will be noticed that the capacity between windings has decreased much faster than the coupling coeificient. thus reducing the degree of amplification t the higher frequencies where th coupling is most effective. his reduct' coupling naturally improves the selectivity over what it would be with tight coupling.

I have also found that the selectivity may still further be improved, with uniform ainplitude of amplification, by reversing the polarity of one winding with respect to the other, as for example, by winding one coil in the opposite direction from the other. In any case, adjacent ends of t 1e two winding; should be at the same potential with respect to ground, high or low, as the case may be. hen wound in opposite directions the natural capacity coupling is in opposite phase to the magnetic coupling of the coils and the spacing between them must be somewhat greater than when wound in the same direction to prevent the two sources of coupling from bucking each other to nearly zero effective coupling at the high frequency end of the range. The greater spacing reduces the capacity coupling so that the mag tic cou pling predominates by an amount .itlicient to produce a substantial degree of amplifiestion at an amplitude which remains practical.- ly constant over the entire operating ran Improvement in selectivity is obtained in t iis case by reason of the greater separation of the windings.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims: the invention itself, however. both as to its organization and method of operation will best be understood by reference to the following description talren in connection with the accompanying drawing in which Figs. 1 and 2- show diagrammaticall two different circuit organizations in which my invention is carried into effect.

I have shown in Fig. 1 receiving system comprising a screen grid radio frequency amplifier device 1, a regenerative detector 2, and two audio frequency amplifying devices 3 and 4:. Si naling currents received upon the antenna a are impressed by means of the coupling transformer 6 upon the tuned input circuit 7 of device 1. The output circuit of device 1 is coupled by means of the coupling transformer 9 through the tuned input circuit 10 of the regenerative detector 2. The output of the circuit of the detector is coupled by means of the transformer 11 to the input circuit of the audio amplifier 3. Conpling transformer 9 is constructed in accordance with the principles which I have de scribed above so as to give a substantially uniform degree of amplification over the entire range over which the receiving set is intended to operate.

In the arrangement shown in Fig. 2, 3 stages of tuned radio frequency amplification employing three electron discharge devices of the screen grid type, 12, 13 and 14:, are provided. The coupling transformers 15, 10 and 17, may all be of the special construction which I have described for giving substantially uniform amplification over the entire operating range.

To pevcnt stray coupling between the different tuned circuits the various screen grid devices and circuits are enclosed in grouna'led shields as indicated, or separator by grounded shields, as shown by dotted lines in the two figures of the drawing. In the arrangement shown in Fig. 2, the d sired operating potential for screening grids 18 is provided by means of suitable source connected terminals 19 and 20. By controlling this potential by means of potentiometer arrangement 21, a convenient and satisfactory volume control of the degree of amplifi-ation provided. This, however, is not a part or my present invention, but is described and claimed in a pending application of C. ill. Burrill, and assigned to the same assignee as the present application.

In the system shown in 1, volume control is accompl' red by means of the v: riablc resistance 22 winch is inserted in the cathode circuit of amplifier 1.

Vlhile I have .own and described only two different circuit organizations in which my invention is employed it will be evident that. many modifications in the circuit arran ments and the specific design of the coupnns transformers may be made without departing from the scole of my invention as set forth in the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is l. The combination in an amu s e tem of a four-electrode screengrid electron discharge device having a tunable input circuit and an output circuit, means for impressing currents to be amplified on the input circuit, and a transfoimer having primary winding in the output circuit and a secondary winding connected in the t able input circuit of a second electron discin gc said transformer having substantially as great a number of prin ary turns secondary turns, and the turns of one winding being wound over the turns of the other winding but separated therefrom to such an extent that the combined capacity d magnetic couplings between the two windings serve to produce substantially uniform voltage air ific 'on over the entire frequency range over wlnch the said input circuits are tunable,

2. The combination in an amplifying sys tem of a four-electrons screen-grid electron. discharge device having a tunable input c'rcuit and an output circuit means for imp ing currents to be amplnuu on n M nu t cult, and a transformer having a p winding in the output circuit and ondary winding connected in the l-i put circuit of a second electron vice, said transformer havi! great a number of primer; ondary turns, the polarity of winding being reve sed with r of the prima I nd sa' ,7 ing spaced to provide a substantially unifo..m tra sfcr of voltage between them over a range of frequencies to which said input circuit is tunable.

The combination in an amplifying systern of a four-electrode screen id electron cuschar e device having a tunable input circurt am. an output circuit, means for impressing currents to be amplified. on the input cuit, and a transformer having a primary winding in the output circuit and a secondary winding connected in the tunable in"; it or;- cuit of a second electron discharge device, said transformer having substantially as number of primary turns as secondar said windings being related that the natural capacity coupling between primary and secondary transfers ei'iergy opposite phase to that transferred by the magnetic c' upliro' and to such an extent that form voltage amplificati A the entire range over which the said input circuits are tunable.

4. The combination in amplifying system of a four-electrode screen-grid electron discharge device having a tunable input cir cuit and an output circuit, means for impressing currents to be amplified on the input circuit, and a transformer having a primary winding in the output circuit and a secondary winding to which said primary is adapted to transfer energy, a tunable input circuit for a second electron discharge device with which circuit said secondary is connected, said transformer having substantially as great a number of primary turns as secondary turns, the polarity of the secondary winding being reversed with respect to that of the primary winding and said winnings being spaced to provide a natural capacity coupling between them which transfers energy in opposite ihase to that transferred by the magnetic coupling existing between said windings and to such an extent that a substantially uniform voltage amplification is produced over the entire frequency range over which the said in put circuit is tunable.

The combination in an amplifying system adapted to operate over a predetermined range of radio frequencies of aplurality of electro d cl'iarge devices having tunable input cl u and a coupling transformer connecting the output circuit of one device to the iput circuit of another device, said transformer having substantially as great a number of primary turns as secondary turns, the primary turns being closely coupled electromagneticaly to the secondary turns and the primary and secondary turns being separated to such an extent by an intervening dielectric that the comb red electromagnetic and electrost. tic couplings serve to produce substantially uniform voltage amplification over the entire frequency range over which the system adapted to operate.

6. The combination in an amplifying sys tem adapted to operate over a predetermined rane of radio frequencies of plurality of eleccron discharge devices having tunable input circuits and a coupling transformer connect ng the output circuit of one device to the input circuit of another device, said transformer having substantially great a number of primary turns as secondary turns, the primary turns being closely coupled electrognetically to the secondary turns and the polarity of the secondary winding being reversed with respect to that of the primary winding, and the primary and secondary turns being separated to such an extent by an ir e muing dielec -c that the combined electroma and electro tatic couplings serve to produce substantially uniform voltage amplification over the entire frequency over which the system is adapted to operate.

In witness whereof, I have hereunto set my hand this 6th day of April, 1928.

BURKE BRADBURY. 

